Magnifying the Past: Galaxy Clusters and Gravitational Lensing (Magnifying, Multiplying, & Distorting Objects in the Distant Early Universe: What It Tells Us) Ray A. Lucas, STScI
Being Single, See(m)ing Double, Triple, Quadruple, … (Multiple images, and Seeing the Same Thing Here and There and There…) Ray A. Lucas, STScI
See Me Now, and See Me Later… (Time Delays, Bending Light, and Seeing the Same Thing Over and Over and Over…) Ray A. Lucas, STScI
Why the Multiple Titles? • I was not trying to torture Frank Summers! (He may have thought so…) • Multi-faceted nature of gravitational lensing: • (1) magnifies the distant objects of the past, • (2) distortions reveal the shape of dark matter halos and gravitational potential by producing multiple images of the same thing, and • (3) illuminates the time-related phenomena that are due to effects of bending/deflecting light-path of distant objects
A few personal notes… • Humility, and my own motivation for the talk… • CL0939+4713 (1994, Dressler, Oemler, Sparks, & Lucas) SM1 ERO, Early WFPC2 “deep” image (~10 orbits, one of the deepest optical images ever taken at the time!), part of inspiration for HDF and its successors… • Abell 2218 (1995, Couch, Ellis, Smail et al.) • Abell 2218 again (1999, Fruchter et al. SM3A ERO) • Abell 370 again (2009, Noll, Chiaberge et al. SM4 ERO)
CL0939+4713 (Abell 851) - 1994, WFPC2, ~10 orbits, V Dressler, Oemler, Sparks, & Lucas, 1994, CL0939+4713, z~0.4, ~4.2 Billion L.Y. One of the deepest images ever taken at the time; it helped inspire the HDF and successors, partly because distant galaxies had higher surface brightnesses than expected, which meant that, completely contrary to predictions, HST was an excellent, in fact consummate, tool for this!
Outline • Gravitational Lensing: various scales, multiple kinds of phenomena (magnification, multiple imaging, and time delays from light-bending) • Some History • Different kinds of examples and their importance • Galaxy Clusters and gravitational lensing (main topic) • Present & Future: New ACS-R ERO; JWST & Beyond
Gravitational Lensing:Some Aspects… • Nature’s gravitational magnifying glass, magnifies images of distant objects • Einstein Rings, arcs, multiple images, distorted images • Bent light, time delays, etc.
Some History: • Einstein predicted bending of light and displacement of images, for example, by gravitational effects of the sun on the apparent positions of more distant stars as seen very near the sun during an eclipse, but first thought it might be too small to be visible on Earth. • Eddington observed this effect during the total solar eclipse of 1919.
Some History (continued) • Einstein’s theory was developed before the true nature of the Milky Way and other galaxies was known, much less the phenomenon of large, massive galaxies and clusters of galaxies. • Massive galaxies and especially clusters of galaxies offered the possibility of seeing this phenomenon on much larger scales than Einstein had originally envisioned.
Einstein, Eddington and the Solar Eclipse of 1919 Eddington, a devout Quaker, felt that proof of the validity of Einstein’s relativity would build a bridge to “German” science, and would help to humanize the recent “enemy” in the wake of WWI. Some claimed that the observations and measurements were not accurate enough at the time, but they were widely accepted as legitimate, and the effect has since been verified. Einstein became famous, and lionized, overnight.
Also from Einstein’s predictions: • - Einstein rings and partial • rings and arcs, double rings, • etc. (above) • Effect also works in other • wavelengths like radio, e.g. • PKS 1830-211 (lower left)
Expansion of the concept… • Microlensing (important for studying stars and finding planets) - Bohdan Paczynski (1980s) • Galaxies can be lenses - Fritz Zwicky (1930s) • Weak lensing (important for studying large-scale extragalactic structure and related alignment/orientation effects) • Single-galaxy strong lensing, Einstein Rings, Einstein’s Cross, etc. - Einstein Rings require axial symmetry of the lensing mass. • Widely-separated identical quasar “pair” • Galaxy Clusters and Lensing (main topic) - strong & weak lensing both play major roles…
Strong, Weak, Micro: Multiple Facets of One Phenomenon… • Manifestation of a given lensing type depends cumulatively upon source geometry & size, lens geometry & mass, distance between the two, & distance and alignment between the sources, the lens, & the observer. It’s a convolution of perspective, geometry, mass, & scale, etc. among all the components. • The same thing seen from a different place or perspective yields a different manifestation.
Microlensing • Positions of distant source stars deflected slightly by presence of nearer intervening planets near lensing stars; positions of more distant stars also deflected slightly by presence of more nearby stars or smaller black holes, etc. as well as/instead of planets. Source magnification varies with position, mass of lensing star, planet, black hole, etc. (Bohdan Paczynski, 1980s)
Weak Lensing Both strong and weak gravitational lensing happens in the field and in galaxy clusters. Rings, arclets and other strongly distorted and magnified features are strongly-lensed, but all distorted objects are at least weakly lensed.
Strong Lensing: Rings, Multiple Images, Arcs Schematic showing comparison of three types of scenarios for strongly-lensed objects. Note that all 3 types of lensing may also exist in the same field of view due to a plethora of sources of various sizes, masses, and types which may intervene between distant source and observer, and the unique geometry which each intervening lensing source has in terms of location with respect to the distant source and the observer.
Strong Lensing (examples) Einstein’s Cross (As seen a few slides earlier…) CL2244-02 (ESO VLT)
First Extragalactic Lens found: Quasar “Pair” Q0957+561 • Discovered by Walsh et al. - 1979 -”Old Faithful” • Separation = 6 arcsec, but spectrally confirmed • Time delay = 417+/-3 days between the two images • Flickering intrinsic, not from intervening objects as previously thought, but still a lensed pair… (L. Goicoechea et al., 2009) Photo credit: A. Ayiomamitis CCD images from P. Young et al., ApJ, 1980
And now, the main topic…! • Galaxy Clusters and Gravitational Lensing: • (1) How does it work? • (2) What is it good for? • (3) What are some limitations of its use? • (4) Some images from HST WFPC2 and ACS imaging… (More soon from WFC3 & ACS! - Large, Multi-Cycle Treasury prop.: Postman, Ford, et al. recently approved - 25 clusters…!) • (5) Abell 370 ACS-R SM4 Early Release Observations: preliminary results; JWST & Beyond.
Galaxy Clusters and Lensing: How Does it Work? (STScI) Bell Labs, Lucent Technologies T. Tyson, G. Kochanski, I. Dell’Antonio F. O’Connell & J. McManus, NY Times • In gravitational lensing: • Convergence term magnifies size • while preserving surface brightness • Shear term stretches images • tangentially; weak lensing stats imp. • Displacement equations may have • multiple solutions = multiple images Want more info? See…: From 1994, a good introductory review by B. Fort & Y. Mellier: http://nedwww.ipac.caltech.edu/level5/Mellier/Mellier_contents.html & http://en.wikipedia.org/wiki/Gravitational_lensing_formalism
Galaxy Clusters and Lensing:How Does it Work? (cont’d) Diagram From Fort & Mellier, 1994 Multiple manifestations of the same phenomenon. Appearance of lensed objects depends on relative alignment of source, lens, and observer, as well as other factors such as distances between source, lens, and observer, and the size of the source, the mass of the lensing object, etc. What you see is just a convolution of all of these factors and more, including surface brightness and color etc. of the source, and other factors as well.
Galaxy Clusters and Lensing • What is it good for? Some principal benefits: • Gives clues to dark matter content (mass) and halo shape of foreground cluster • Magnifies distant galaxies in early universe, aiding studies of galaxy formation, morphological structure (shapes), stellar populations, dust & metals content, etc. (i.e. revealing early star-forming history & rates) • Enables study of supernovae etc. at much higher redshifts, giving clues to cosmology • Time delays, AGN flickering --> cosmology
Galaxy Clusters and Lensing • What else is it good for? • Makes beautiful, amazing images, showing us just how rich Nature can be in its array of phenomena, and that it also works in amazing but normal ways that are actually predictable. • Scientists are not immune to the power of beautiful images. We love them too, and they are part of our inspiration - part of what attracted us to go into science, to discover the beauty of how something looks AND how it works!
Galaxy Clusters and Lensing: Some Limitations • Degeneracies between lensing effects and cosmological time dilation effects can make interpretation of high-z SNe results more difficult. • Need highest resolution and largest number of multiply-imaged sources to derive best constraints on mass models of clusters and to properly reconstruct morphologies of distant objects. • Multiple colors needed for best matching of source IDs of multiply-lensed objects, etc.
Early Ground-based Observations and Discoveries • Abell 370 (Genevieve Soucail et al., CFHT, mid-1980s: One of the first lensing clusters found and ID’d) - We’ll return to this one later! • Others joined in, as well… • Some fundamental questions raised: Tidal tails or gravitationally-lensed arcs? Near or far? Two, three, four etc. objects or just one seen over and over and over…? Time delays if multiply- imaged objects, AGN flickering, supernovae, etc.? Implications for distance scale and cosmology?
NGC4038/9: “Antennae” Tidal Tails HST - B. Whitmore et al. Ground-based • Tidal tails sometimes mimic the appearance of gravitationally-lensed arcs, or vice-versa, even with very high-resolution HST data! • A case in point: New ACS/WFC Abell 370 images! (We’ll return to that later… But first, two older ones of Abell 370, taken from the ground…)
Abell 370 (CFHT, mid-1980s) Image: CFHT Prime Focus Image: CFHT, B. Fort & Y. Mellier • Lynds & Petrosian (1986) and Soucail et al. (1987) point out existence of large curved arcs around two clusters of galaxies • Paczynski (1987) announces correct interpretation… • Soucail et al. (1988) spectroscopically confirm that redshift of A370 arc much greater than that of cluster --> lensing in clusters confirmed!
Abell 370 Spectrum Soucail et al., 1988, Using ESO 3.6m Telescope + EFOSC/PUMA2 Spectrograph • Oxygen II [OII] line confirms the nature of the large arc as a galaxy. • Presence of the [OII] with same redshift all along the arc indicates that it is the same galaxy, distorted, and stretched out into an arc, possibly imaged multiple times in the same arc.
Abell 370 - CFHT (mid-1980s) • Spectra confirm that giant arc is the same more distant galaxy magnified and imaged multiple times. The basic phenomenon is confirmed via essential spectroscopy! But other cases still questionable… Could some still just be tidal tails? Of course…! But most in the vicinity of massive galaxy clusters and galaxies are usually lensed arcs… Spectra tell the tale… Phot-z’s used if too faint… • Spec/Phot-z evidence critical; mass models and lensing equations help untangle things…
Abell 370 - Distant galaxy at z~6.56) - It’s the tiny object! From ApJ Letters, Hu et al., 2002 E. Hu et al., 2002, strong emission-line object discovered in Keck LRIS Narrowband image, left-most in the panels above.
HST and Lensing Clusters • Wide-field high-resolution capabilities of first WFPC2 (1994) and later ACS/WFC (2002) provide richer data for interpretation over wider fields. (Ground-based adaptive optics only yield high-resolution over very small area… Wide area high-resolution is critical!) • + • Nature’s gift (distant galaxies have higher surface brightnesses than expected) • = A bonanza of great new HST data of observations of lensing clusters!
HST NICMOS, WFC3, and Spitzer Multiwavelength Obs. • Some of the highest-redshift objects are not even found in ACS images, but require data from the infrared + optical to identify. They are “dropouts” from bluer light & filters due to cosmological expansion, i.e. redshift. • NICMOS and Spitzer have provided near-IR and “farther” IR observations to detect such “dropouts”; WFC3 will do so in near-IR in the future (now), as well! (JWST will probe farther into the IR, like Spitzer, but with better sensitivity and resolution.)
“Dropouts” and “Phot-z’s”: Important in Field & Clusters Galaxies “drop out” of view from UV towards red and infrared filters etc., depending on how distant they are and thus how far their light is redshifted. This is also shown relative to the filter response or throughput curves for HST WFPC2 Ultraviolet (U), Blue (B), Visual (V), and long red (I) filters. Many very distant objects are too faint to observe spectroscopically. The time required would be too long even with massive ground-based telescopes like Keck. So, by observing in many well-chosen and well-calibrated filters, and then making careful note of how objects drop out of them at successively redder wavelengths, photometric redshifts are obtained. With use of proper “priors”, these are also now understood to be reliable indicators of the redshift and therefore the distance of sources in both field surveys as well as distant lensed galaxies found behind galaxy clusters. Graphic courtesy of Mark Dickinson.
HST and Lensing Clusters • Some prime examples… (Just a very few of the many clusters and lenses imaged by HST, and only a small sample of the work done - by no means a comprehensive review of all the work done on these objects - many apologies to those not mentioned here!): • Abell 2218, Abell 1689, Abell 1703, Bullet Cluster, CL2244-02, CL0024+1654, MACS Cluster J0025.5-1222, etc., and Abell 370 again, after SM4…
Abell 2218, HST+WFPC2 circa 1995 A2218 by W. Couch, R. Ellis, I. Smail et al., ~1995 • Single filter (i.e. one broad-band color); but still spectacular! • Showed richness of high-resolution detail, but not as much about fine details of colors of objects. • Also maybe one of the only programs ever to be taken totally from end-to-end in hands-on fashion by one STScI contact person… ;-)
Abell 2218 (ca. 1999-2000) SM3A ERO A. Fruchter and the SM3A ERO Team Abell 2218 z~0.176 or ~2 Billion l.y. Distant
Abell 2218 (Details) - Most Distant Galaxy, 2004 Kneib, Ellis et al., 2004, Distant galaxy, at z~7.0, ~13 billion yrs old, “pair” of red objects are same object encircled twice; Orange arc = E gal @ z~0.7; Blue Galaxies = star-forming galaxies at z~1-2.5.
Abell 1689 Abell 1689, z~0.183, ~2.2 Billion l.y. Distant Largest Einstein radius known - ~50 arcsec. (Einstein radius is the angular size of an Einstein Ring, if such were present.) Abell 1689 also represents the largest number of strong-lensing constraints (multiply-lensed sources, etc.) in one field of any galaxy cluster known. This means the mass model can be more accurate.
Abell 1689 - HST ACS+NICMOS & Spitzer Line of infinite magnification; Magnification increases as you approach this line from either side, and there is a trough of lower magnification between this and the line around the nucleus of the cluster. The magnification factor of the distant lensed galaxy in the circle at left Is a factor of about 9x. L. Bradley et al., ApJ, 2008 ACS/WFC image; Blue = NICMOS J, Orange = NICMOS H H-band image is used to verify that its not a dusty, lower-redshift object masquerading as high-z object.
Abell 1689 - A Newer Most Distant Galaxy - 2008 Abell 1689, z~0.183, 2.2 Billion l.y.; Galaxy A1689-zD1, z~7.6, ~12.97 Billion l.y. This object is a “dropout” from bluer wavelengths, hence only visible in the infrared. This also illustrates the importance of multiwavelength observing. More distant objects have since been found, e.g. a GRB at z~8 and some z~8-8.5 galaxies in Bouwens, Illingworth, and Stiavelli’s new HUDF GOODS-South WFC3 deep “boreholes” recently announced in January 2010.